This book goes a long way towards explaining that. And in many ways, this book stands apart from his music, and can be enjoyed without ever hearing or knowing of Cage's music pieces. Because the music was almost by accident - Schoenberg told Cage that he was an inventor, not a composer, and this book demonstrates that, and goes further to show Cage was a philosopher. Music just happened to be the medium where he best expressed his philosophy, but it could have been painting or film, depending on his path. The book defines a way of living and thinking and seeing, and of course hearing, the world. That's what it's about. And it's beautiful and gentle quality capture the essence of Cage, a true quiet revolutionary. His revolution was profound, and best expressed in his piano piece 4'33", where the pianist does not make a sound at the instrument. The revolution of that event was the most profound and destabilizing in the history of music, and yet it was entirely silent. Such is the power of Cage's ideas that he has no need to really 'lecture' about them, he merely presents them and let's their own strength do the rest.
There's a common argument that his ideas (and this book) are overrated. I find this difficult to digest, especially when one considers the enormous impact Cage's writings and compositions have had on countless composers (basically anyone composing after 1950 has most likely taken a thing or two from the ideas in this book).
Sometimes he can be a little tough to follow in the book, as properly constructed sentences are not high up on Cage's list of priorities. However, this book has so much to offer that it is worth wading through the occasional slow spot.
So give it a whirl. Even if you don't like Cage's music, reading this book will give you insights into what he did that may change your mind or at least instill a newfound respect. At its best, this is inspiration of the highest sort.
Romantic? Yes, I would say that for instance his idea of "sounds in themselves" and "nature" are romantic. Can we really eliminate all cultural impact and distortion just by refusing intention? I think not. Sounds are always inflected by history.
Still, I would not want a world without the challenge of his extreme stance.
It's just a remarkable text.
You have to get it.